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The reproductive success of songbirds is lower in years with surges of tree seed production (masting), suggests a new study by Northeast CASC-supported authors, as increased seed availability expands the range of the red squirrel, a dominant nest predator.

Every few years, many northeastern tree species synchronously produce significantly higher numbers of seeds, nuts, and cones than usual – an occurrence known as masting. This dramatic surge of seed production introduces a pulse of resources to ecosystems that can have rippling effects on species’ interactions. 


Northeast CASC Research Ecologist Toni Lyn Morelli and a team of researchers published a study examining how masting events in northeastern forests impact red squirrels, which are a primary nest predator of multiple migratory songbird species. Using decades of seed surveys collected by scientists and community scientists, squirrel surveys, and observations of over 500 bird nests, the researchers connected tree masting, squirrel population dynamics, and songbird nest survival rates.  


The study highlights that after masting events of American Beech and Balsam Fir across New England and New York, red squirrel populations not only increase from the added resources, allowing them to survive winter better and reproduce more successfully, but they also expand their range into higher elevation spruce-fir dominated forests. This range expansion means that the squirrels encounter more bird species that nest across a wider range of elevations, which allows for increased predation. The squirrel population boom and range expansion were related to lower reproductive success of the migratory songbird species examined in the study, including the Bicknell’s Thrush and the Blackpoll Warbler, which are already facing climate change threats and experiencing population declines. Interestingly, the authors note that red squirrels seem to anticipate masting events and expand higher in elevation before masting occurs. During non-masting years, the red squirrels remain primarily in the transition zone between lower elevation hardwood forest and higher elevation spruce-fir forest.  


This Northeast CASC-supported research offers important insights into the relationships between tree masting events, red squirrel populations, and songbird reproduction, informing northern forest management and songbird conservation in the Northeast.  


This work was supported by the Northeast CASC Project titled "Increasing the Resilience and Resistance of Climate-Vulnerable Species and Ecosystems." 

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